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figure 16–5. A patient prepared for a cVEMP recording
using frontal skull taps.
tial reflex hammer that generates a trigger pulse when
the hammer strikes the head. The pulse triggers a recording epoch and the signal averager records the response.
The mechanical stimulus is an iner-
hearing loss. In a similar study, Yang and Young (2003)
compared the clinical utility of skull taps and air-conducted stimuli for recording cVEMPs in patients with
otitis media. As expected, only 59% of the subjects with
middle ear disease generated measurable cVEMPs in
response to the air-conducted stimuli compared with
91% when light skull taps were used. The patients who
failed to generate responses to skull taps had larger
conductive impairments. Mechanical stimulation also
has been shown to be useful in identifying peripheral
vestibular end-organ impairments. Brantberg and colleagues (2003) evaluated cVEMP responses using forehead skull taps and air-conducted stimuli in patients
diagnosed with vestibular neuritis. Cervical VEMPs
recorded using air-conducted clicks and skull taps
were similar. It is noteworthy that the location (e.g.,
lateral versus forehead) of the skull taps can produce
different responses. Brantberg and Tribukait (2002)
showed that taps to the forehead produced bilateral
cVEMP waveforms similar to those generated by airconducted stimuli. However, cVEMPs in response to
lateral skull taps generated a typical looking cVEMP
(i.e., P1 to N1) from the contralateral side but an antiphasic waveform from the ipsilateral side. The authors
suggested that this bilateral response represented synchronized EMG activity from the SCMs analogous to
what would occur during a natural translation of the
head. Figure 16–6 shows cVEMP responses obtained
in the author’s lab using a customized “skull tapper”
delivered to the midline of the skull.
recording technique. The most common method is to
record the response bilaterally, since both the left and
right saccules are stimulated by the mechanical stimulus. The non-inverting and inverting electrode inputs
are unchanged. However, the ground electrode must be
moved elsewhere, since the forehead is the site where
the skull taps are delivered. The response is recorded
with the patient supine and with a gauze 4 × 4 cm pad
placed on the forehead. The patient is asked to lift his/
her head in the midline position and to push against the
examiner’s hand that is exerting a gentle force against
the patient’s head. This creates an isometric contraction
of the bilateral SCMs and tonic EMG activity that will
be attenuated by the mechanical stimulus. Halmagyi
et al. (1995) showed that cVEMP responses generated
with mechanical stimulation could be recorded bilaterally, had larger amplitudes than air-conducted stimuli,
and were measurable in the presence of conductive
figure 16–6. A cVEMP recorded in response to frontal
skull taps using a novel bone tapper device (Intelligent
Hearing Systems). Note: x-axis is time in ms.
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Stimulus Frequency
The effect of stimulus frequency on the cVEMP has
been systematically described by several groups of
investigators (Akin, Murnane, & Proffitt, 2003; Murofushi et al., 1999; Piker, Jacobson, Burkard, McCaslin,
& Hood, 2013). The consensus of these investigations
and others has been that although the cVEMP can
be recorded using unfiltered click stimuli, maximum
cVEMP amplitude is obtained using short-duration,
low-frequency tone bursts. In one of the first studies
describing the effects of stimulus frequency on the
cVEMP, Murofushi and colleagues (1999) compared
cVEMP amplitudes in response to low-frequency tone
bursts and clicks. The frequency-specific stimuli were
500, 1000, and 2000 Hz tone bursts. The authors showed
responses recorded with 500-Hz tone bursts and clicks
were similar in amplitude, while the 2000 Hz cVEMPs
were the smallest (Figure 16–7). Similarly, Todd, Cody,
and Banks (2000) investigated the differences in the
amplitude of cVEMPs recorded with 100-, 200-, 400-,
800-, 1600-, and 3200-Hz tone bursts while keeping the
intensity stable (100 dB SPL). The authors applied a
curve-fitting algorithm to the data and determined that
the stimulus frequency that maximized cVEMP amplitude was between 300 and 650 Hz. Akin et al. (2003)
also investigated the effect of stimulus frequency (i.e.,
500–750 Hz) on latency and threshold of the cVEMP
response. Changing the frequency of the stimulus does
not appear to have an effect on latency when the rise
and fall time of the stimulus remained constant (Akin
et al., 2003; Welgampola & Colebatch, 2001a). Welgampola and Colebatch (2001a) recorded cVEMP responses
using tone bursts between 200 and 1000 Hz using 100Hz increments. The investigators reported that cVEMPs
with the largest amplitudes were generated in response
to stimuli between 600 and 1000 Hz. The finding that
low-frequency tone bursts between 500 and 1000 Hz
are the optimal auditory stimuli to generate a cVEMP
has been replicated by numerous groups of investigators (Akin et al., 2003; Lin et al., 2006; Murofushi et al.,
1999). These findings are in agreement with neurophysiological recordings from the inferior vestibular
nerve afferents in cats where tuning is most sensitive
from 500 to 1000 Hz (McCue & Guinan, 1995).
Stimulus Intensity
The level of the stimulus used to elicit a cVEMP response
directly influences the amplitude of the cVEMP. Recording a cVEMP requires a high-intensity stimulus with a
short onset time (e.g., 95 to 100 dB normalized hearing
Figure 16–7. The effect of stimulus frequency on cVEMP
amplitude. stimulus level was 127 dB p
on the left represent individual runs and the recordings
on the right represent the average of responses. From
Piker, E.
and Hood,
cVEMP and oVEMP. Ear and Hearing, 34(6), 65–73. Used
with permission.
G., Jacobson, G. P., Burkard, R. F., McCaslin, D. L.,
L. J. (2013). Effects of age on the tuning of the
SPL. The tracings
level [nHL], with a 1 cycle rise/fall time) (Colebatch et al.,
1994; Ochi, Ohashi, & Nishino, 2001). In fact, stimulus
intensities near or below 75 dB HL are not sufficient
to generate a cVEMP in most individuals with normal
vestibular function (Akin et al., 2003; Papathanasiou,
Murofushi, Akin, & Colebatch, 2014). Most commercial evoked potential systems that are used to record
cVEMPs are capable of generating stimuli of sufficient
intensity to consistently record cVEMPs. However, it
is important to ensure that the system generating the
stimulus is calibrated using peak-to-peak equivalent
SPL so that peak and cumulative sound exposure can
406 BALANCE FUNCTION ASSESSMENT AND MANAGEMENT
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be measured and calculated (Papathanasiou et al.,
2014; Rosengren, Govender, & Colebatch, 2009). The
stimulus generator should be routinely calibrated in
dB peak SPL using a sound-level meter. Stimulus level
is one of the key factors that modulate the amplitude of
the cVEMP response. The relationship between oVEMP
amplitude and intensity is maintained with click stimuli
as well as frequency-specific stimuli such as tone bursts
(Akin et al., 2003; Colebatch et al., 1994; Ochi et al.,
2001). In a comprehensive look at the effect of stimulus
level on cVEMP amplitude, Ochi and colleagues (2001)
reported significant changes in the amplitude of the
cVEMP with increasing stimulus level (i.e., 85, 95, and
105 dB). Specifically, the authors reported peak-to-peak
amplitudes of 203.96 microvolts (µV), 264.10 µV, and
293.35 µV for cVEMPs evoked using clicks at 95, 100,
and 105 dB nHL, respectively. However, this relationship between stimulus intensity and cVEMP amplitude is not completely linear. Figure 16–8 illustrates
the effect of stimulus intensity on cVEMP amplitude.
Stimulus Rate
The effects of presentation rate/stimulus presentation
rate on the cVEMP have been described (Brantberg &
Fransson, 2001; Wu & Murofushi, 1999). Brantberg and
Fransson (2001) presented stimuli using presentation
rates of 4, 6, 8, and 20 per second. The authors evaluated waveform reproducibility and used a specific set
of criteria to register whether a cVEMP response was
present or absent. In order for a response to be classified as present, the peak-to-peak amplitude of the
cVEMP had to significantly exceed the voltage variation of the first 5 ms following stimulus onset. Furthermore, the initial component of the cVEMP needed to
be both identifiable in the 15 to 20 ms post-stimulus
period and reproducible. The authors reported that as
stimulus rate was increased from 6 to 20 Hz, cVEMP
responses that met the preset criterion decreased from
87% (6/sec) to 56% (20/sec). Furthermore, the cVEMP
peak-to-peak amplitude was shown to decrease significantly as stimulus rate was increased.
In a similar study, Wu and Murofushi (1999) evaluated the effects of five repetition rates (1, 5, 10, 15, and
20 Hz) on the response characteristics of the cVEMP.
They reported that responses could be recorded using
repetition rates of 10 Hz and less. The cVEMP was
recorded in only 63% of subjects when the rate of
stimulus presentation was 20 Hz. The investigators
also reported that the largest VEMP amplitudes were
recorded using stimulation rates of 5 Hz and below
Figure 16–8. The effect of stimulus level on cVEMP amplitude.
16. VESTIBULAR-EVOKED MYOGENIC POTENTIALS (VEMPS) 407
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(Figure 16–9). Based on this investigation, the authors
recommended a stimulus repetition rate of 5 Hz to
maximize amplitude and reproducibility.
Stimulus Gating and Duration
The envelope of a tone burst refers to how the onset
and offset of the stimulus are shaped. Different gating
parameters can change the spectral characteristics of
a frequency-specific stimulus. Cheng and Murofushi
(2001a) evaluated the effect of stimulus rise/fall time
on the latency of the VEMP. The investigators reported
their observations of the effects of four rise and fall
times on the latency of the P1 and N1, and the time
separating the two peaks. The investigators reported
that P1 latency increased as the rise/fall time increased.
A similar trend of increasing latency with increasing
stimulus rise and fall time was reported for N1.
Welgampola and Colebatch (2001a) evaluated the
optimal duration for frequency-specific stimuli using
durations of 1, 3, 5, 7, 10, and 20 ms delivered at a rate
of five per second. The investigators reported that the
largest cVEMP responses were obtained when stimuli
of approximately 7 ms duration were used.
Stimulus Monaural/Binaural
The clinical utility of monaural versus binaural stimulation for recording cVEMPs has been described.
Yang and Young (2003) described the characteristics of
cVEMPs in response to monaural and binaural stimula-
tion. The authors were particularly interested in determining if cVEMP latencies, amplitudes, and response
rates recorded using binaural stimulation were similar to those recorded when two monaural responses
were recorded and analyzed separately. In fact, there
were no significant differences found when response
metrics were compared between the two stimulation
paradigms. The authors concluded that it was appropriate to use bilateral stimulation and recordings. Bilateral testing has the potential to reduce the amount of
recording time by 50%. This procedure has merit where
patients may be unable to sustain the required muscle
contraction for the time it would take to record two
monaural recordings (e.g., elderly patients).
RECORDING VARIABLES
Electrode Placement
Cervical VEMP recording requires that the non-inverting electrode (i.e., active electrode) be placed at the
midpoint between the termination of the muscle at the
mastoid and its origin at the sternum. The resulting
response is a positive peak (P1) followed by a negative
peak (N1) (Jacobson & McCaslin, 2007). Sheykholeslami, Murofushi, and Kaga (2001) examined the effect
of electrode position on cVEMP amplitude and latency
by recording from several different locations along the
length of the SCM (Figure 16–10). The investigators
showed that response latency was most stable when
the response was recorded from the belly (i.e., middle)
Figure 16–9. The effect of stimulus rate (500 Hz tone burst) on cVEMP response amplitude.
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Figure 16–10. Electrode locations used to determine
the optimal placement for recording the c
middle part of the
tion for recording c
Murofushi, T., and Kaga, K. (2001). The effect of sternocleidomastoid electrode location on vestibular-evoked
myogenic potential. Auris Nasus Larynx, 28(1), 41–43.
Used with permission.
SCM muscle is the optimal loca-
VEMPs. From Sheykholeslami, K.,
VEMP. The
of the muscle. The inverting electrode (i.e., reference)
should be placed in an electrically indifferent position. Several investigations have reported placing the
inverting electrode near the sternal tendons or at the
sternoclavicular junction (Rosengren, Welgampola, &
Colebatch, 2010). However, in our experience, these
locations are not always free from reference contamination. The common or ground electrode can be affixed
to the forehead except when a mechanical stimulus
is employed. This electrode configuration results in
positive potentials represented as upward deflections.
The cVEMP response is predominantly a unilateral
response, although in some instances contralateral
responses have been shown to be present. Accordingly, a one- or two-channel evoked potential system
is appropriate for recording the response because only
the ipsilateral response is typically measured for diagnostic purposes.
Amplification and Filtering
The cVEMP is much larger than a typical neurogenic
auditory evoked potential. Where an auditory brain-
stem response (ABR) wave V might be 0.5 µV, a cVEMP
P1 response may be 300 µV in amplitude. This means
that amplification values of only 3000 to 5000× are necessary to bring the response into the recording range of
the signal averager. Because the response is a stimulus
synchronized attenuation of tonic EMG activity, it is
necessary to ensure that artifact rejection is disabled.
If artifact rejection is disabled, it is incumbent on the
examiner to monitor the amplifier input to the signal
average to ensure that saturation of the amplitude has
not occurred (i.e., clipping of the raw EMG). Where
amplifier saturation has occurred, it is a simple matter
to reduce the amplifier gain (e.g., 5000× to 3000×).
There has been variability in the reported optimal bandpass filter (BPF) settings (Burkard, McCaslin,
Jacobson, & McNeerney, 2010; McCaslin, Jacobson,
Hatton, Fowler, & Delong, 2013; Ochi et al., 2001;
Vanspauwen, Wuyts, & Va de Henning, 2006). BPF is
a process that enables the clinician to reject unwanted
endogenous (e.g., EKG) and exogenous (e.g., 60 Hz)
electrical interference (Jones et al., 2002). The decision of how wide to set the BPF to record the response
of interest is based on (1) the spectral characteristics
of the response, (2) the spectral characteristics of the
unwanted noise, and (3) the “skirt” of the filter (Wang,
Jaw, & Young, 2013). In an effort to describe the optimal BPF for the cVEMP, Burkard, McCaslin, Jacobson,
and McNeerney (2010) recorded cVEMPs from eight
subjects. VEMPs were obtained using 120 dB peak (p)
SPL 500 Hz tone bursts (2-1-2 cycle, Blackman window)
and presented at a rate of 5 Hz (Jacobson & McCaslin,
2007). Responses were averaged to ~250 stimuli, and
each response was replicated one time. The authors
reported that the dominant energy composing the
cVEMP response was in the range of 15 to 70 Hz. Therefore, the authors recommended a minimum high-pass
cutoff of ~5 to 15 Hz and a minimum low-pass cutoff
of ~100 to 150 Hz.
SUBJECT VARIABLES
EMG Activity and Monitoring
Even though the absolute latency of P1 and interaural
P1 latency differences are routinely measured during
the cVEMP recording, amplitude has become the standard parameter for detecting most abnormalities affecting the end organs in clinical populations (Jacobson
& McCaslin, 2007; McCaslin et al., 2013). Intersubject
absolute amplitude variability is an issue that arises
when using cVEMP amplitude in clinical assessments.
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One method to account for the intersubject amplitude variability is to use relative amplitude measures
through the use of amplitude asymmetry calculations.
A confounding factor associated with using amplitude
measures is the known relationship between the tonic
background EMG and the peak-to-peak amplitude of
the cVEMP. In this regard, Lim, Clouston, Sheean, and
Yiannikas (1995) reported that cVEMP amplitude was
positively correlated with the level of tonic muscle
activity (Lim et al., 1995). That is, as the level of tonic
EMG increases, the amplitude of the response increases
(Akin et al., 2004; Karino et al., 2005; Lee et al., 2008;
Lim et al., 1995; McCaslin et al., 2014; Welgampola &
Colebatch, 2001b; Figure 16–11). Because the cVEMP
represents a synchronized attenuation of tonic EMG
activity, patients must activate the SCM (i.e., produce a
criterion level of tonic EMG above the resting baseline)
in order to resolve a response (i.e., which is the soundsynchronized reduction in background EMG). Sev-
eral methods have been described for the purpose of
maximally activating the SCM during cVEMP recording. Two proven methods described in the literature are
(1) superior flexion of the head while rotating the head
away from the ear stimulated with the patient in a semirecumbent position (Figure 16–12) and (2) lifting the
head at midline while the patient is in the supine position (i.e., bilateral activation; Figure 16–13) (Colebatch et
al., 1994; Rosengren et al., 2010; Vanspauwen et al., 2006;
Wang & Young, 2006; Zapala & Brey, 2004). The former
technique has been reported to consistently generate
cVEMP in normal participants and is the technique we
currently employ in our laboratory (Isaradisaikul et al.,
2008; McCaslin et al., 2013; Wang & Young, 2006).
Accounting for the level of SCM activation is critical for both the reliability of cVEMP measures as well as
validity of interaural measures. Occasionally, patients
are unable to generate equal amounts of background
EMG for testing the left and right sides (e.g., patients
UncorrectedP1–N1Amplitude
figu re 16 –11. cVEMP amplitude increases with increases in EMG
amplitude. Each subject was instructed to keep their tonic EMG as
close to a fixed point as possible on the monitor. From McCaslin,
D. L., Fowler, A., and Jacobson, G. P. (2014). Amplitude normalization
reduces cervical vestibular evoked myogenic potential (cVEMP)
amplitude asymmetries in normal subjects: Proof of concept. Jour-
nal of the American Academy of Audiology, 25(3), 268–277. Used with
permission.
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figure 16–12. A model prepared for a two-channel cVEMP recording
using the sternoclavicular junction as the inverting (reference) electrode. The head is rotated away from the ear stimulated while in the
semirecumbent position.
with cervical spondylosis). Two primary methods have
been shown to be valid for use in controlling for the
level of background tonic EMG activity. These include:
(1) self-monitoring of the EMG activity by the patient
through the use of a visual EMG target (Colebatch &
figure 16–13. A model prepared for a two-channel cVEMP recording using the sternoclavicular junction as the inverting (reference)
electrode. The head is lifted in the midline while the patient is in the
supine position. This technique can be used for bilateral activation
and recording.
Halmagyi, 1992) and (2) mathematical correction (i.e.,
amplitude normalization) of evoked potential amplitude for the magnitude of EMG that occurred during
signal averaging (Brantberg et al., 2008; McCaslin et al.,
2014; Welgampola & Colebatch, 2001b).
16. VESTIBULAR-EVOKED MYOGENIC POTENTIALS (VEMPS) 411
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Patient Self-Monitoring (Biofeedback)
Self-monitoring of EMG involves the use of biofeedback. That is, the patient is provided an ongoing
measure of his or her rectified continuous tonic EMG
activity. The EMG is transformed into a visual target
and displayed on a screen for the patient to observe
in real time during the recording. In most instances,
EMG magnitude is represented on the vertical axis
and recording time displayed along the horizontal axis
(Figure 16–14).
This form of feedback enables the patient to compare his or her current level of EMG with the preset
target level window (i.e., minimum and maximum
amount of allowable EMG). The patient must continually maintain a specified level of EMG activity and
either increase or decrease his or her ongoing EMG
activity to stay within the predefined target window.
When the level of EMG in a particular recording epoch
exceeds the ceiling of the window or falls below the
minimum allowable EMG, the individual sweep is
rejected. In this way the variability of the EMG is controlled to a degree that the tester predetermines. Viewing the ongoing EMG activity during the recording also
provides the patient with a threshold to exceed in order
to ensure that an adequate level of SCM contraction
is achieved to produce a cVEMP (e.g., 50 µV). What
must be known before a target window for the EMG
can be created is what the variability is when a subject
is asked to maintain a certain level of EMG at a target
level (McCaslin et al., 2014). In order to answer this
question, McCaslin and colleagues (2014) instructed
study participants to contract their SCMs in such a
way as to maintain the level of EMG at one of four target levels (i.e., of 100, 200, 300, and 400 µV). A visual
target was provided via a video monitor for the subjects to observe. At the end of each recording, a mean
(and standard deviation [SD]) of the EMG activity was
calculated. EMG variability (i.e., window width) was
calculated using two SDs from the mean. Figure 16–15
shows that as the EMG target increases, background
muscle activity variability increases and the window
should be widened (McCaslin et al., 2014). Because
of the variability in commercial EMG monitors, it is
recommended that normative data be obtained that
quantify the variability associated with different target
levels before setting the target window width.
There is now consensus that controlling for the
level of EMG during a cVEMP recording is critical (Akin
et al., 2004; Akin, Murnane, Tampas, & Clinard, 2011;
McCaslin et al., 2014). In one of the first articles demonstrating the relationship between EMG and cVEMP
figure 16–14. A representative EMG monitor (Interacoustics) for recording cVEMPs. EMG
magnitude is represented on the vertical axis and time represented on the horizontal
axis. The percent completed dial allows the patient see how much time remains for the
recording.
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figure 16–15. Mean RMS EMG variability increases with increases
MG amplitude. Each subject was instructed to keep his or her
in E
tonic E
MG as close to a fixed point as possible on the monitor. The
upper ends of the boxplots represent the 75th percentile (upper
quartile), and the lower ends of the boxplots represent the 25th
percentile (lower quartile).
by the line in the center of the boxplot. From
A., and Jacobson,
cervical vestibular evoked myogenic potential (c
asymmetries in normal subjects: Proof of concept. Journal of the
American Academy of Audiology, 25(3), 268–277. doi:10.3766/
jaaa.25.3.6.
Used with permission.
G. P. (2014). Amplitude normalization reduces
The median of the data is represented
amplitude, Akin and associates (2004) systematically
described how cVEMP amplitude increased with corresponding increases in EMG. The authors employed
an EMG monitor to quantify the amplitude of the background muscle activity and concluded that the optimal amount of tonic EMG for recording a cVEMP fell
between 30 and 50 µV. The results of the study concluded that the close relationship between EMG and
cVEMP amplitude makes monitoring the EMG during
a cVEMP recording necessary. Without the ability to
monitor background SCM EMG, the cVEMP responses
from the left and right sides cannot be reliably compared. However, some investigators have failed to find
statistically significant differences in grouped data
between unmonitored and self-monitoring conditions
when optimal muscle activation techniques were used
(Isaradisaikul et al., 2008; McCaslin et al., 2013). Isaradisaikul and associates (2008) acknowledged that there
McCaslin, D. L., Fowler,
VEMP) amplitude
were a number of patients who did benefit from monitoring (although it did not reach statistical significance)
and suggested that EMG monitoring is beneficial for a
subset of patients receiving a less than perfectly administered test procedure (e.g., in the case of a severely
asymmetrical SCM activation).
cVEMP Amplitude Normalization
A second technique for controlling for the effects of
asymmetrical tonic EMG activity during the cVEMP
recording is through the use of a mathematical correction known as amplitude normalization (Colebatch et al.,
1994; Lee, Cha, Jung, Park, & Yeo, 2008; McCaslin et al.,
2013, 2014). Colebatch and colleagues (1994) described
a method to correct for asymmetrical muscle contraction that utilized a calculation of the magnitude of
the rectified EMG that occurred in the pre-stimulus
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period. The method consists of collecting a sample of
tonic EMG activity preceding the stimulus onset (e.g.,
100 ms in duration) during each recording epoch and
then calculating the mean root mean square (RMS)
value of the rectified pre-stimulus EMG to derive an
average. This pre-stimulus EMG average theoretically
represents an estimate of the EMG produced by the
contraction of the SCM during the overall recording.
A derived waveform that accounts for the level of EMG
during the recording is then calculated by dividing the
mean RMS of the EMG into each data point of the final
signal averaged cVEMP waveform. In this way the
amplitude of P1–N1 is “normalized” so that side-toside amplitude comparisons can be calculated and the
variability of side-to-side differences in muscle contraction (reflected by EMG amplitude) can be controlled
(Figure 16–16).
Several investigators have studied the clinical
utility of amplitude normalization with mixed results
(Bogle, Zapala, Criter, & Burkard, 2013; Kim, Jung, Lee,
& Suh, 2013; McCaslin et al., 2013). McCaslin and associates (2013) reported that in a group of normal subjects
(i.e., pediatric and adult) amplitude normalization did
not reduce significantly the variability in the interaural
amplitude asymmetry when a single EMG target was
employed. In some instances, amplitude normalization converted an “abnormal” cVEMP into a “normal”
cVEMP, although the opposite effect occurred as well,
suggesting that the amplitude normalization technique
was valuable in a subset of patients in their sample but
not enough to reach statistical significance. In a followup study designed to further investigate the effectiveness of amplitude normalization on asymmetrical
EMG, the investigators recorded cVEMPs while having
patients maintain four different levels of EMG amplitude (i.e., 100, 200, 300, and 400 µV) for each ear. This
method enabled the investigators to compare cVEMP
responses obtained with varying levels of background
EMG and evaluate the effectiveness of amplitude normalization. For each ear in each condition the P1–N1
interaural asymmetry ratios were calculated. As was
expected, when responses obtained using dramatically different EMG target levels were compared (e.g.,
100 µV target versus 400 µV target), large asymmetries
were noted (Figure 16–17). Following the application
of amplitude normalization, cVEMP amplitude did
not change significantly with changes in RMS EMG or
EMG target levels for any condition (see Table 16–2).
This study confirmed the benefits of using amplitude
normalization as well as helped determine the degree
of tonic EMG asymmetry required to generate an abnormal amplitude asymmetry result in normal subjects.
One fact that has come to light recently is that
the relationship between EMG amplitude and P1–N1
amplitude is not entirely linear. That is, the input-output
growth function for cVEMP peak-to-peak amplitude
has been shown to saturate at supramaximal SCM contraction levels (Bogle et al., 2013; McCaslin et al., 2014).
The point where further increases in background SCM
EMG do not yield corresponding increases in P1–N1
figure 16–16. Example of a subject with asymmetrical EMG during the cVEMP recording and whose interaural
asymmetry value was transformed by amplitude normalization.
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